In the control of systems having rotating drive shafts, torque and speed are the fundamental parameters of interest. Therefore, the sensing and measurement of torque in an accurate, reliable and inexpensive manner has been a primary objective of workers for several decades.
Previously, torque measurement was accomplished using contact-type sensors directly attached to the shaft. One such sensor is a "strain gauge" type torque detection apparatus, in which one or more strain gauges are directly attached to the outer peripheral surface of the shaft and a change in resistance caused by strain is measured by a bridge circuit or other well known means. However, contact-type sensors are relatively unstable and of limited reliability due to the direct contact with the rotating shaft. In addition, they are very expensive and are thus commercially impractical for competitive use in many of the applications, such as automotive steering systems, for which torque sensors are now being sought.
Subsequently, non-contact torque sensors of the magnetostrictive type were developed for use with rotating shafts. For example, U.S. Pat. No. 4,896,544 to Garshelis discloses a sensor comprising a torque carrying member, with an appropriately ferromagnetic and magnetostrictive surface, two axially distinct circumferential bands within the member that are endowed with respectively symmetrical, helically directed residual stress induced magnetic anisotropy, and a magnetic discriminator device for detecting, without contacting the torqued member, differences in the response of the two bands to equeal, axial magnetizing forces. Most typically, magnetization and sensing are accomplished by providing a pair of excitation or magnetizing coils overlying and surrounding the bands, with the coils connected in series and driven by alternating current. Torque is sensed using a pair of oppositely connected sensing coils for measuring a difference signal resulting from the fluxes of the two bands. Unfortunately, providing sufficient space for the requisite excitation and sensing coils on and around the device on which the sensor is used has created practical problems in applications where space is at a premium. Also, such sensors appear to be impractically expensive for use on highly cost-competitive devices, such as in automotive applications.
More recently, torque transducers based on measuring the field arising from the torque induced tilting of initially circumferential remanent magnetizations have been developed which, preferably, utilize a thin wall ring ("collar") serving as the field generating element. See, for example, U.S. Pat. Nos. 5,351,555 and 5,520,059 to Garshelis. Tensile "hoop" stress in the ring, associated with the means of its attachment to the shaft carrying the torque being measured establishes a dominant, circumferentially directed, uniaxial anisotropy. Upon the application of torsional stress to the shaft, the magnetization reorients and becomes increasingly helical as torsional stress increases. The helical magnetization resulting from torsion has both a circumferential component and an axial component, the magnitude of the axial component depending entirely on the torsion. One or more magnetic field vector sensors sense the magnitude and polarity of the field arising, as a result of the applied torque, in the space about the transducer and provides a signal output reflecting the magnitude of the torque. The stability of this transducer's "torque-to-field" transfer function under rigorous conditions of use reflects the efficacy of uniaxial anisotropy in stabilizing circular polarizations. This anisotropy, together with the spatially closed nature of the quiescent polarization, is also the basis of a striking immunity from polarization loss in relatively large fields. While the fields that arise from the ring itself have only hard axis components relative to the anisotropy, "parasitic" fields from permeable material that is close enough to become magnetized by the ring field have no such limitation. The addition of such parasitic fields to the torque dependent field from the ring can seriously degrade the near ideal features of the transfer function. As a result, in order to avoid a major source of such distortion, either the underlying shaft, or a sleeve that is placed between the shaft and the ring, is generally fabricated from a paramagnetic material. In addition, inasmuch as the peak allowable torque in a ring sensor is limited by slippage at the ring/shaft interface, concerns have been expressed regarding distortion arising from slippage at the ring/shaft interface under conditions of torque overload. This need for multiple parts of different materials, together with the requirement that the methods and details of their assembly establish both a rigid, slip-free mechanical unit and a desired magnetic anisotropy, have encouraged the investigation of alternative constructions.